Political boundaries are the criterion scholars use most often to define American suburbs; a problematic approach when applied to the late nineteenth century. Annexation distended the boundaries of nineteenth-century cities so far as to obscure broad swaths of suburban and rural districts within their limits. The absence of a literature about these “suburbs in the city” is problematic, because it encourages historical researchers to consider newly annexed territory as urban equivalents of older city districts. This article argues that under the generally accepted definition of suburb, the condition of nineteenth-century urban overbounding obstructs a full appreciation of the historical breadth, ubiquity, and composition of working-class residence outside the urban core. Analysis of the socioeconomic characteristics of regions with suburban population densities within the 1880 city limits of Cleveland, Ohio; Milwaukee, Wisconsin; Newark, New Jersey; and St. Louis, Missouri, indicate that researchers have underestimated the degree and diversity of blue-collar suburbanization during this period.